OK I must write this down lest I should ask Hilaire for help the third time about this simple task. For my poor eyes, the Dr. Geo sketches do not look very clear in the default dark background. This is how you change the background to white: click on the workspace background to bring up the system menu. Open the "settings" dialog. Click on "style". Change the "User Interface Theme" to "Light theme". The change takes effect immediately. Now you have a white background.

To make the change permanent, again bring up the system menu Then click "tools" => "Save Session".

Choosing the light theme